The present invention relates to radio telephone communications. Specifically, a power amplifier used in a portable telephone is described for either analog or digital cellular communications.
The development of the cellular telephone service has undergone significant technological changes. The earlier AMPS system for communicating by portable telephones utilizes a frequency modulation system so that voice signals were carried to a base station as a frequency modulated radio frequency signal. The frequency modulation system has the advantage of providing efficient radio frequency power generation which is important in battery operated devices. The portable radio telephone includes a radio frequency power amplifier implemented in bipolar transistor technology, and radio frequency transmit signals are amplified at a maximum transistor efficiency. To obtain the maximum efficiency, the output transistors are operated in the saturation region.
A newer wireless communications format, CDMA, has been introduced with different operating requirements. CDMA power amplifiers operate in a linear mode with slightly less power. Many AMPs power amplifiers can be modified for CDMA operation by simply changing the load impedance.
Current wireless telephones receive both AMPs and CDMA operations so as not to render the earlier system obsolete. One of the problems in providing both modes of operation in a radio telephone, which can be selected depending upon the cell system in which the radio telephone is used, is that the transmit power amplifier is usually designed for one or the other mode of operation. In the CDMA mode, amplitude information must be preserved and therefore, a high linearity must be maintained in the amplification stages. This will effectively reduce the total output power that would be otherwise attainable by operating the amplifier in the saturation mode. On the other hand, however, operating the amplifier in a linear mode when the AMPS frequency modulation mode is used for communication, unnecessarily reduces the efficiency of the amplification stages reducing the battery life.
One approach to avoiding the foregoing difficulty of operating an amplifier in either a saturated, or a linear mode is disclosed in U.S. Pat. No. 5,060,294. A dual mode power amplifier is described which changes the bipolar transistor amplification stage of a radio telephone from a saturated mode of operation, useful for the AMPS modulation system, to a highly linear mode of operation when a CDMA transmission format is desired. While the concept of a dual mode power amplifier is useful for this application, significant drawbacks occur when implementing the amplifier in the cellular telephone art. These drawbacks originate from unstable VSWR conditions which can change the operating mode of the power amplifier, significantly degrading the amplifier performance, particularly when operating in the linear mode. The present invention is directed to providing a dual mode amplifier which is not disturbed by the changing VSWR conditions of the radio telephone antenna.
In accordance with the invention, a balanced power amplifier is provided which may be operated in a switched mode. A directional coupler receives on one port a signal to be amplified. A first power amplifier is connected to a second port of the directional coupler, and amplifies the signal in either a high linearity mode, or in a saturation mode as selected by the user. A second power amplifier is connected to a third port of the directional coupler and receives the remaining half of the signal received by the first port, and also operates in a switched mode, operating either in the high linearity mode or in the saturation mode of operation.
Output signals from the first and second power amplifiers are recombined in a second directional coupler. The second directional coupler has an output port which may be connected to an output load such as an antenna for a cellular telephone, and prevents the transmission of reflected power from the antenna to the output of each amplifier.